1. Field of the Invention
The present invention relates to a heat fixing device equipped on an image forming apparatus such as an electrophotographic copying machine and an electrophotographic printer.
2. Description of the Related Art
An image forming apparatus such as a copying machine and a printer of an electrophotographic system is equipped with a heat fixing device that fixes a toner image on a recording material. When a toner image is printed on a small-sized recording material, temperature in a region through which the recording material does not pass in the heat fixing device is excessively elevated. As one measure for the temperature to be excessively elevated, a technique in which a cooling fan that cools a paper non-passing region is provided is available.
In Japanese Patent Application Laid-Open No. 2007-187816, a cooling unit (a cooling fan) that cools the paper non-passing region of a heating member is provided to reduce excessive temperature elevation in the paper non-passing region. In this method, it is possible to reduce the excessive temperature elevation by arranging a temperature detection element in the paper non-passing region of the heating member and positively sending a cooled wind in an air volume depending on the detected temperature. This method can also meet recording materials having various widths by changing a cooled region depending on the width of the recording material.
However, this method has a problem in that some wind sent from the cooling fan to the paper non-passing region to be elevated in temperature in the heating member enters the inside of a paper passing region to drop the temperature in edges of the paper passing region in the heating member (temperature drop). In particular, in a printer in which a large number of sheets are printed per unit period of time, the temperature in the paper non-passing region is largely elevated. Thus, it is necessary to increase the air volume of the cooled wind in order to keep the temperature of the heating member equivalent to or lower than an upper temperature limit. However, since the air volume is large, some of the wind also lowers the temperature in the edges of the paper passing region in the heating member. This deteriorates a fixability and causes an uneven gloss in both edges of the small sized recording material. Thus, to maintain the fixability, the air volume in the cooling fan cannot be increased simply.
For this problem, in Japanese Patent Application Laid-Open No. 2009-288275, after driving the cooling fan, an energization ratio of multiple heating devices with different heating distribution is controlled so that a heat volume in a region corresponding to a region cooled by the cooling fan is increased compared with that before being cooled. However, in Japanese Patent Application Laid-Open No. 2009-288275, the heat volume in the region to be cooled is increased, and thus, wasted electricity is consumed. Also when the number of sheets to be printed per unit period of time is further increased, the temperature in the paper non-passing region is further elevated. To deal with this excessive temperature elevation, a cooling fan with high power is required, easily leading to increasing in size of the apparatus.